Unconformities and Porosity Development in Carbonate Strata: Ideas from a Hedberg Conference

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Publication Date

1-1-1994

Publication Title

AAPG Bulletin

Volume Number

78

Issue Number

6

Abstract

The purpose of this paper is to highlight ideas from a Hedberg Conference concerning detection of unconformities and associated porosity in carbonate strata. Many types of information, including cores and outcrops, seismic data, eustatic sea level curves, wireline logs, biostratigraphic data, stable isotope trends, cycle stacking patterns, and tectonic and basin evolution models, can be used to predict and/or detect subaerial unconformities. All methods of detecting subaerial exposure with these types of information have limits and pitfalls. Detailed analysis of the stratigraphy, sedimentology, and diagenetic history of a rock sequence from outcrop or core data is considered the most reliable means of detecting subaerial exposure, though an integrated approach using all a ailable information is recommended. The relationship between subaerial exposure and subsurface porosity in carbonates is not simple or easily predicted. Subsurface porosity is the product of many factors operating during deposition, subaerial exposure, and burial. Several concepts should be recognized when evaluating porosity associated with subaerial unconformities. (1) Carbonate diagenesis during subaerial exposure rearranges pore networks, but usually does not increase the total porosity. Near-surface dissolution can simply lower a topographic surface without increasing porosity within the host limestone. In many cases, total porosity in carbonates under unconformities is actually reduced during subaerial exposure. (2) Permeability is likely to change more than porosity during subaerial exposure, increasing in some c ses but decreasing in others. (3) Pore systems evolve with time during subaerial exposure. Short periods of subaerial exposure (10,000-400,000 yr) are often associated with greater porosity than long intervals of subaerial exposure (1-20 million yr). Prolonged subaerial exposure may change permeability less than porosity because high-permeability, karst-related conduits can form quickly and persist for millions of years. (4) Many carbonates subjected to subaerial exposure have little or no porosity in the deep subsurface because compaction, cementation, and stratal collapse reduce porosity during burial. (5) Unconformity-related diagenesis may enhance reservoir potential by creating pore systems that are resistant to compaction during deeper burial. (6) Vugs, caverns, and breccias can fo m in the deep subsurface because of dissolution by basinal fluids independent of subaerial exposure. (7) Lithologic changes at unconformities, like shales overlying carbonates, can influence the flow of subsurface fluids during

Keywords

Carbonates, Unconformities (Geology)

Document Type

Article

Digital Object Identifier (DOI)

https://doi.org/10.1306/A25FE3C9-171B-11D7-8645000102C1865D

Language

English

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